The Dosimetric Impact of Inter-fractional Organ-at-Risk Movement During Liver Stereotactic Body Radiation Therapy

Abstract Purpose Stereotactic body radiation therapy (SBRT) is an effective therapy for treating liver malignancies. However, little is known about interfractional dose variations to adjacent organs-at-risk (OARs). We examine the effects of interfractional organ movement and setup variation on dose delivered to OARs in patients receiving liver SBRT. Methods and Materials Thirty subjects treated with liver SBRT were analyzed. Daily image-guidance with diagnostic quality CT-on-rails imaging was performed prior to each fraction. In Phase I, these daily images were used to delineate all OARs including the liver, heart, right kidney, esophagus, stomach, duodenum, and large bowel in 10 patients. In Phase II, only OARS in close proximity to the target were contoured in 20 additional patients. Dose distribution on each daily CT was generated and daily doses to each OAR were recorded and compared to clinical thresholds to determine whether a daily dose excess (DDE) occurred. Results Phase 1: Significant interfractional dose differences between planned and delivered dose to OARs were observed, but differences were rarely clinically significant with just one DDE. Phase 2: multiple DDEs were recorded for OARs close to the target mainly involving the stomach, heart, and esophagus. Tumors in the hilum, liver segments I, IV, and VIII were the most common locations for DDEs. On root cause analysis, three etiologies of DDE emerged, Cranial-Caudal shift (69.2%), anatomic changes (28.2%), and Anterior-Posterior shifts (2.6%). Conclusions OARs close to liver lesions may receive higher doses than expected during SBRT due to interfractional variations in OARs relative to the target. These differences in planned versus expected dose can lead to toxicity. Efforts to better evaluate OARs with daily image guidance may help reduce risks. Application of adaptive replanning and improved and real time image guidance could mitigate risks of toxicity and further study into their applications is warranted.